fix(i18n): localize broker cluster instance selector

[yyqdbngt]

Summary

The Studio BrokerCluster page toolbar instance selector still used hard-coded zh for its accessible name and placeholder.

Localized in pages/studio/BrokerCluster.tsx:

• The select aria-label and placeholder (选择实例) now use the new brokerCluster.selectInstance key.
• 1 new brokerCluster.* key; zh byte-identical.

Why

The toolbar selector is the primary instance switch on the page; its accessible name must follow the active language.

Testing

• tsc --noEmit passes
• eslint clean on the changed files

[RockteMQ-AI]

Summary

Replaces hardcoded Chinese strings in BrokerCluster.tsx with i18n translation keys. Clean and straightforward fix.

LGTM — good catch on the missing i18n keys.

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